A new series of objectives under the name “Digiprime” has been offered by the Zeiss company. The objectives are for high definition digital cameras. Such cameras are offered, for example, by the Sony or Panasonic companies.
An essential difference from the objectives for film cameras heretofore consists of the possibility of changing the image-side intercept distance (termed “back focus” in English) of the objective by means of a rotary ring. This is necessary because the tolerance for the back focus within the camera is large in comparison with a film camera. Moreover, the objective holder in the camera is as a rule made of aluminum, and greater wear can be expected. Such an adjustment can of course be achieved in that the back focus of the camera is changed, or a back focus change takes place by means of an intermediate ring inserted between objective and camera.
The adjustment to the correct back focus must take place very accurately, since a faulty setting results in out of focus pictures.
Methods are described hereinafter which are up to now known for matching the back focus of the objective to the back focus of the camera.
In methods used in practice up to now, a Siemens star printed on cardboard is imaged from a distance of 1–2 m by means of the camera, which is provided with a corresponding objective. The distance of the Siemens star from the objective or the camera is measured with a tape measure. The image of the Siemens star obtained with the camera is assessed. For adjustment, the image is made sharp by turning the back focus adjusting ring. The image can be observed during this adjustment by means of the viewfinder, or else by an external control monitor.